The Fisher-Tropsch (F-T) synthesis, the core process in the gas-to-liquids (GTL) technique, originates from the preparation of synthetic fuel from syngas by coal gasification invented by German chemists Fischer and Tropsch in 1923. The GTL process is composed of the three major sub-processes of (1) reforming of natural gas, (2) F-T synthesis of syngas and (3) up-grading of product. The F-T reaction which is performed at a reaction temperature of 200 to 350° C. and a pressure of 10 to 30 atm using the iron- and cobalt-based catalysts can be described by the following four key reactions.
(a) Chain Growth in F-T SynthesisCO+2H2→—CH2—+H2OH(227° C.)=−165 kJ/mol
(b) MethanationCO+3H2→CH4+H2OH(227° C.)=−215 kJ/mol
(c) Water Gas Shift ReactionCO+H2OCO2+H2H(227° C.)=−40 kJ/mol
(d) Boudouard Reaction2COC+CO2H(227° C.)=−134 kJ/mol
For the F-T reaction, mainly iron- and cobalt-based catalysts are used. In early times, iron-based catalysts were preferred. But, recently, cobalt catalysts are predominant in order to increase the production of liquid fuel or wax and to improve conversion. Iron-based catalysts are characterized in that they are the most inexpensive F-T catalysts producing less methane at high temperature and showing high selectivity for olefins and the product can be utilized as a starting material in chemical industry as light olefin or α-olefin, as well as fuel. In addition, a lot of byproducts such as alcohols, aldehydes and ketones, etc., are produced concomitantly with the formation of hydrocarbons. And, the iron-based catalyst are mainly used in the low-temperature F-T reaction for wax production by Sasol which is composed of Cu and K components as cocatalyst and is synthesized by precipitation method using SiO2 as binder. And, Sasol's high-temperature F-T catalyst is prepared by melting magnetite, K, alumina, MgO, etc. The price of cobalt-based catalyst is about 200 times higher than that of Fe catalysts. But, it shows a higher activity, longer lifetime and higher liquid paraffin-based hydrocarbon production yield with lower CO2 generation. However, they can be used only at low temperature because CH4 is produced dominantly at high temperature. And, since the expensive cobalt precursor is used, the catalysts are prepared by dispersing on a stable support with a large surface area, such as alumina, silica, titania, etc. And, a small amount of precious metals such as Pt, Ru and Re, etc., is added as cocatalyst.
The mechanism by which the main product, or the straight-chain hydrocarbons, is produced is mainly explained by the Schulz-Flory polymerization kinetic scheme. In the F-T process, more than 60% of the primary product has a boiling point higher than that of diesel oil. Thus, diesel oil can be produced by the following hydrocracking process and the wax component can be transformed into high-quality lubricant base oil through the dewaxing process.
In general, the current reforming process of atmospheric residue or vacuum residue used in the refinery plant is a reliable one owing to the improvement of catalysts and processing techniques. However, for the F-T synthetic oil, further development of an adequate hydrocarbon reforming process is required, because there is a big difference in compositions and physical properties from the source material used in the refinery plant. Examples of the processes for treating the primary product of the F-T reaction include hydrocracking, dewaxing, isomerization, allylation, and so forth. And, major products of the F-T reaction include naphtha/gasoline, middle distillates with a high centane number, sulfur- and aromatic-free liquid hydrocarbons, α-olefins, oxygenates, waxes, and so forth.
Typically, in order to disperse high-priced active components, cobalt or other activation substance is introduced to a support having a large surface area, such as alumina, silica, titania, etc., to prepare a catalyst. Specifically, in the F-T reaction, a catalyst prepared by dispersing cobalt on a single-component or multi-component support is commercially utilized. However, if the particle size of the cobalt included in the support is similar, the activity of the F-T reaction does not change a lot from one support to another [Applied Catalysis A 161 (1997)59]. On the contrary, the activity of the F-T reaction is greatly affected by the dispersion and particle size of cobalt [Journal of American Chemical Society, 128 (2006) 3956]. Accordingly, a lot of attempts are being made to improve the FTS activity and stability by modifying the surface property of the supports by pre-treating them with different metal components.
For instance, when cobalt-supported alumina is used, the surface characteristics of γ-alumina may be transformed into, for example, that of boehmite because of the water produced during the reaction. As a result, the catalyst may become inactivated or thermal stability may be reduced due to the increased oxidation rate of the cobalt component support. In order to solve this problem, provided is a method of improving the stability of the catalyst by pretreating the surface of alumina using a silicon precursor [WO 2007/009680 A1]. Also, a method of treating an alumina support with a structural stabilizer including various metals such as magnesium, zirconium, barium, boron, lanthanum, etc. in order to improve hydrothermal stability is proposed [U.S. Pat. No. 7,071,239 B2].
As another method of improving the activity of the F-T catalyst, a method of improving the catalyst stability by increasing the transfer rate of the compounds having a high boiling point produced during the F-T reaction and by preparing a silica-alumina catalyst having a bimodal pore structure is reported [US 2005/0107479 A1; Applied Catalysis A 292 (2005) 252].
However, the aforementioned methods are associated with the complicated processes to synthesize the support with a bimodal pore structure using a polymer substrate or physically mixing two alumina-silica gels prepared so as to have different pore sizes and then supporting cobalt or other active component.
In case silica is used as support, decreases of reduction to cobalt metal and consequent reduction of activity are observed due to the strong interaction between cobalt and the support, as compared with the alumina support. It is reported that pretreating the silica surface with zirconium or other metal is effective in overcoming this problem [EP 0167215 A2; Journal of Catalysis 185 (1999) 120].
The aforesaid F-T catalysts show various specific surface areas, but the activity of the F-T reaction is known to be closely related with the particle size of the cobalt component, pore size distribution of the support and reducing tendency of the cobalt component. To improve these properties, a preparation method of the F-T catalyst by including the cobalt component through a well-known method on the support prepared through a complicated process is reported.